The present invention relates to manufacturing parts by molding thermoplastic material, and more particularly but not exclusively, to manufacturing parts having a surface appearance that is pearly or metallic.
To confer a pearly appearance to parts made of molded plastics material, it is known to incorporate an appropriate filler in the plastic.
Nevertheless, the flow lines of the material during molding usually remain visible on the part made in that way, which can be esthetically unpleasing.
Furthermore, the presence of the filler can give rise to problems with the part being fragile.
Also, to give a metallic appearance to parts made of molded plastics material, it is known to subject them to vacuum metallization or to immerse them in an electrolytic bath, but those techniques are relatively expensive to implement.
It is known that parts made of plastics material can be painted by spraying paint or varnish thereon, however that solution remains relatively expensive and can also give rise to environmental problems.
WO 95/13177 discloses a method in which the plastics material is initially injected into a mold cavity, and then the parts constituting the mold are moved relative to each other so as to leave a gap into which paint can be injected. That method requires a mold that is relatively complicated since the displacement of the component parts of the mold must be controlled accurately prior to injecting paint.
FR-A-2 729 886 discloses the method in which the plastics material is initially injected and then, before the core of the part has cooled down completely, paint is injected. The molded part is painted without opening the mold. The still-fluid core of the part is compressible and allows the paint which is injected under pressure to spread around the part. That method is relatively difficult to implement since the capacity for compression of the plastics material depends on the thickness of the non-solidified layer in the core of the part. The shape of the part must therefore be implemented accordingly, thereby complicating design.
The present invention seeks in particular to facilitate the manufacture of parts made of plastics material and that have a special surface appearance, e.g. pearly or metallic.
The invention achieves this by a novel method of making a painted part out of molded thermoplastic material, the method comprising the steps consisting in:
making the part to be painted by a method of molding thermoplastic material in the cavity of a mold;
allowing the part made in this way to cool at least on the surface, cooling being accompanied by shrinkage, and being performed by opening the mold that was used to make the part that is to be painted; and
injecting paint around the part in a cavity of a mold having the same shape as the mold cavity that was used for molding the thermoplastic material.
In order to simplify the description, the term xe2x80x9cpaintxe2x80x9d should be understood here and below and in the claims as also extending to varnishes.
By means of the invention, it is possible to obtain a pearly or other surface appearance without any need to add a filler to the thermoplastic material, thereby solving the problems of traces left by the flow lines of the material.
The shrinkage of the thermoplastic material during cooling provides a space inside the mold cavity around the molded part into which paint can be injected.
In addition, the invention makes it possible to remedy the problems of fragility induced by the presence of a filler within the thermoplastic material.
The paint used can also serve to increase scratch resistance by being harder than the plastics material constituting the core of the part.
It is thus very easy to make a part having the looked-for surface appearance by using an appropriate paint.
In particular, it is possible to use a metallized paint and to obtain a molded part having a metallic appearance without it being necessary to perform vacuum metallization.
Furthermore, because of the invention, paint is deposited around the molded part without the environmental problems that are encountered when paint is sprayed.
In a particular implementation of the invention, the part is cooled by being extracted from the mold.
The molded part can be subjected to surface treatment prior to paint being injected, e.g. corona effect treatment to improve the adhesion of the paint on the molded part.
In a particular implementation of the method of the invention, the mold is rotated at least through part of a turn relative both to a feed member for feeding thermoplastic material and to a feed member for feeding paint.
Preferably, the mold has a first set of cavities in which molding of the thermoplastic material takes place, and a second set of cavities in which the paint is injected around the previously-molded part.
In a variant implementation of the method, after the thermoplastic material has been molded, the molded parts are extracted from the mold and transferred to a different mold, where the different mold nevertheless has cavities of the same shape as the cavities that were used for molding said parts.
In a particular implementation of the invention, a paint is used that is cured under the action of ultraviolet radiation, and after the paint has been injected, the mold is opened and the painted part is exposed to ultraviolet radiation.
In a variant implementation of the method, a paint is used that is cured under the action of heat, and after the paint has been injected, it is heated to a temperature higher than its curing temperature.
Preferably, the paint is injected at a temperature less than or equal to 100xc2x0 C., and preferably the paint is injected cold.
It is also possible to cure the paint by exposing it to an appropriate curing agent, e.g. a peroxide.
The paints used can be based on polyurethane or on epoxy and can contain metallic fillers.
The invention also provides a painted molded part obtained by implementing the above-specified method.
The part can be constituted by a thermoplastic material coated on its surface in a layer of paint deposited by being injected into the mold, and of thickness lying in the range 10 xcexcm to 150 xcexcm, and preferably in the range 25 xcexcm to 50 xcexcm.
By way of example, the painted molded part can constitute a flask, a stopper, or a box.
The invention also provides apparatus for manufacturing a painted molded part, the apparatus comprising:
a first set of mold cavities;
a second set of mold cavities, the cavities of said first and second sets being identical in shape;
an injector member for injecting thermoplastic material in the molten state into the first set of mold cavities; and
an injector member for injecting paint into the second set of mold cavities around parts previously made by means of the first set of mold cavities.
Compared with spray deposition, the invention makes it possible to reduce paint losses since the paint can be deposited with precisely the required thickness.
In addition, the invention makes it possible to manufacture painted molded parts using a small number of operations and molds that are relatively simple.
When the parts made are flasks or the like, the invention makes it possible to avoid certain problems of chemical compatibility that are encountered with prior art molded parts containing fillers that are not compatible with the composition that is to be contained inside the flask.
The invention makes it possible to use a thermoplastic material that has no filler or coloring agent, that is entirely compatible with the composition that is to be contained inside the flask, while nevertheless obtaining the looked-for external appearance, since there is no risk of the pigments or coloring agents in the paint used coming into contact with the above-mentioned composition.